Voltage regulation is commonly required to prevent variation in the supply voltage powering various microelectronic components such as digital ICs, semiconductor memory, display modules, hard disk drives, RF circuitry, microprocessors, digital signal processors and analog ICs, especially in battery powered application likes cell phones, notebook computers and consumer products.
Since the battery or DC input voltage of a product often must be stepped-up to a higher DC voltage, or stepped-down to a lower DC voltage, such regulators are referred to as DC-to-DC converters. Step-down converters are used whenever a battery's voltage is greater than the desired load voltage. Step-down converters may comprise inductive switching regulators, capacitive charge pumps, and linear regulators. Conversely, step-up converters, commonly referred to boost converters, are needed whenever a battery's voltage is lower than the voltage needed to power its load. Step-up converters may comprise inductive switching regulators or capacitive charge pumps.
Of the aforementioned voltage regulators, the inductive switching converter can achieve superior performance over the widest range of currents, input voltages and output voltages. The fundamental principal of a DC/DC inductive switching converter is based on the simple premise that the current in an inductor (coil or transformer) cannot be changed instantly and that an inductor will produce an opposing voltage to resist any change in its current.
The basic principle of an inductor-based DC/DC switching converter is to switch or “chop” a DC supply into pulses or bursts, and to filter those bursts using a low-pass filter comprising an inductor and capacitor to produce a well behaved time varying voltage, i.e. to change DC into AC. By using one or more transistors switching at a high frequency to repeatedly magnetize and de-magnetize an inductor, the inductor can be used to step-up or step-down the converter's input, producing an output voltage different from its input. After changing the AC voltage up or down using magnetics, the output is then rectified back into DC, and filtered to remove any ripple.
The transistors are typically implemented using MOSFETs with a low on-state resistance, commonly referred to as “power MOSFETs”. Using feedback from the converter's output voltage to control the switching conditions, a constant well-regulated output voltage can be maintained despite rapid changes in the converter's input voltage or its output current.
To remove any AC noise or ripple generated by switching action of the transistors, an output capacitor is placed across the output of the switching regulator circuit. Together the inductor and the output capacitor form a “low-pass” filter able to remove the majority of the transistors' switching noise from reaching the load. The switching frequency, typically 1 MHz or more, must be “high” relative to the resonant frequency of the filter's “LC” tank. Averaged across multiple switching cycles, the switched inductor behaves like a programmable current source with a slow-changing average current.
Since the average inductor current is controlled by transistors that are either biased as “on” or “off” switches, then power dissipation in the transistors is theoretically small and high converter efficiencies, in the eighty to ninety percent range, can be realized. Specifically when a power MOSFET is biased as an on-state switch using a “high” gate bias, it exhibits a linear I-V drain characteristic with a low RDS(on) resistance typically 200 milliohms or less. At 0.5 A for example, such a device will exhibit a maximum voltage drop ID·RDS(on) of only 100 mV despite its high drain current. Its power dissipation during its on-state conduction time is ID2·RDS(on). In the example given the power dissipation during the transistor's conduction is (0.5 A)2·(0.2Ω))=50 mW.
In its off state, a power MOSFET has its gate biased to its source, i.e. so that VGS=0. Even with an applied drain voltage VDS equal to a converter's battery input voltage Vbatt, a power MOSFET's drain current IDSS is very small, typically well below one microampere and more generally nanoamperes. The current IDSS primarily comprises junction leakage.
So a power MOSFET used as a switch in a DC/DC converter is efficient since in its off condition it exhibits low currents at high voltages, and in its on state it exhibits high currents at a low voltage drop. Excepting switching transients, the ID·VDS product in the power MOSFET remains small, and power dissipation in the switch remains low.
Power MOSFETs are not only used to convert AC into DC by chopping the input supply, but may also be used to replace the rectifier diodes needed to rectify the synthesized AC back into DC. Operation of a MOSFET as a rectifier often is accomplished by placing the MOSFET in parallel with a Schottky diode and turning on the MOSFET whenever the diode conducts, i.e. synchronous to the diode's conduction. In such an application, the MOSFET is therefore referred to as a synchronous rectifier.
Since the synchronous rectifier MOSFET can be sized to have a low on-resistance and a lower voltage drop than the Schottky diode, conduction current is diverted from the diode to the MOSFET channel and overall power dissipation in the “rectifier” is reduced. Most power MOSFETs include a parasitic source-to-drain diode. In a switching regulator, the orientation of this intrinsic P-N diode must be the same polarity as the Schottky diode, i.e. cathode to cathode, anode to anode. Since the parallel combination of this silicon P-N diode and the Schottky diode only carry current for brief intervals known as “break-before-make” before the synchronous rectifier MOSFET turns on, the average power dissipation in the diodes is low and the Schottky oftentimes is eliminated altogether.
Assuming transistor switching events are relatively fast compared to the oscillating period, the power loss during switching can in circuit analysis be considered negligible or alternatively treated as a fixed power loss. Overall, then, the power lost in a low-voltage switching regulator can be estimated by considering the conduction and gate drive losses. At multi-megahertz switching frequencies, however, the switching waveform analysis becomes more significant and must be considered by analyzing a device's drain voltage, drain current, and gate bias voltage drive versus time.
Based on the above principles, present day inductor-based DC/DC switching regulators are implemented using a wide range of circuits, inductors, and converter topologies. Broadly they are divided into two major types of topologies, non-isolated and isolated converters.
The most common isolated converters include the flyback and the forward converter, and require a transformer or coupled inductor. At higher power, full bridge converters are also used. Isolated converters are able to step up or step down their input voltage by adjusting the primary to secondary winding ratio of the transformer. Transformers with multiple windings can produce multiple outputs simultaneously, including voltages both higher and lower than the input. The disadvantage of transformers is they are large compared to single-winding inductors and suffer from unwanted stray inductances.
Non-isolated power supplies include the step-down Buck converter, the step-up boost converter, and the Buck-boost converter. Buck and boost converters are especially efficient and compact in size, especially operating in the megahertz frequency range where inductors 2.2 μH or less may be used. Such topologies produce a single regulated output voltage per coil, and require a dedicated control loop and separate PWM controller for each output to constantly adjust switch on-times to regulate voltage.
In portable and battery powered applications, synchronous rectification is commonly employed to improve efficiency. A step-down Buck converter employing synchronous rectification is known as a synchronous Buck regulator. A step-up boost converter employing synchronous rectification is known as a synchronous boost converter.
Synchronous Boost Converter Operation: As illustrated in FIG. 1, prior art synchronous boost converter 1 includes a low-side power MOSFET switch 4, battery connected inductor 5, an output capacitor 8, and “floating” synchronous rectifier MOSFET 7 with parallel rectifier diode 6. The gates of the MOSFETs driven by break-before-make circuitry 3 and controlled by PWM controller 2 in response to voltage feedback VFB from the converter's output present across filter capacitor 8. Break-before-make, i.e. BBM, operation is needed to prevent shorting out output capacitor 8.
The synchronous rectifier MOSFET 7, which may be N-channel or P-channel, is considered floating in the sense that its source and drain terminals are not permanently connected to any supply rail, i.e. neither to ground or Vbatt. Diode 6 is a P-N diode intrinsic to synchronous rectifier MOSFET 7, regardless whether synchronous rectifier is a P-channel or an N-channel device. A Schottky diode may be included in parallel with MOSFET 7 but with series inductance may not operate fast enough to divert current from forward biasing intrinsic diode 6. Diode 9 comprises a P-N junction diode intrinsic to N-channel low-side MOSFET 4 and remains reverse biased under normal boost converter operation. Since diode 9 does not conduct under normal boost operation, it is shown as dotted lines.
If we define the converter's duty factor D as the time that energy flows from the battery or power source into the DC/DC converter, i.e. during the time that low-side MOSFET switch 4 is on and inductor 5 is being magnetized, then the output to input voltage ratio of a boost converter is proportionate to the inverse of 1 minus its duty factor, i.e.
            V      out              V      in        =            1              1        -        D              ≡          1              1        -                  (                                    t              SW                        T                    )                    
While this equation describes a wide range of conversion ratios, the boost converter cannot smoothly approach a unity transfer characteristic without requiring extremely fast devices and circuit response times. For high duty factors and conversion ratios, the inductor conducts large spikes of current and degrades efficiency. Considering these factors, boost converter duty factors are practically limited to the range of 5% to 75%.
The Need for Dual Polarity Regulated Voltages: Today's electronic devices require a large number of regulated voltages to operate, some of which may be negative with respect to ground. Some smart phones may use more than twenty-five separate regulated supplies in a single handheld, including negative bias supply needed for some organic light emitting diodes (OLEDs), displays, for biasing LCD's, and for a variety of other applications. Space limitations preclude the use of so many switching regulators each with separate inductors.
Unfortunately, multiple output non-isolated converters capable of generating both positive and negative supply voltage require multiple winding or tapped inductors. While smaller than isolated converters and transformers, tapped inductors are also substantially larger and taller in height than single winding inductors, and suffer from increased parasitic effects and radiated noise. As a result multiple winding inductors are typically not employed in any space sensitive or portable device such as handsets and portable consumer electronics.
As a compromise, today's portable devices employ only a few switching regulators in combination with a number of linear regulators to produce the requisite number of independent supply voltages. While the efficiency of the low-drop-out linear regulators, or LDOs, is often worse than the switching regulators, they are much smaller and lower in cost since no coil is required. As a result efficiency and battery life is sacrificed for lower cost and smaller size. Negative supply voltages require a dedicated switching regulator that cannot be shared with positive voltage regulators. More than one negative regulated supply voltage may be required.
What is needed is a switching regulator implementation capable of producing both multiple positive and negative outputs, i.e. multiple dual polarity outputs, from a single winding inductor, minimizing both cost and size.